Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation Abstract State-of-the-art climate models predict the zonal mean mid-latitude circulation will undergo a poleward shift and seasonally and hemispherically dependent intensity changes in the future. Here I review the mechanisms put forward to explain the zonal mean mid-latitude circulation response to increased carbon dioxide (CO2) concentration. The mechanisms are grouped according to their thermodynamic starting point, which are thought to arise from processes independent of the zonal mean mid-latitude circulation response. There are 24 mechanisms and 8 thermodynamic starting points: (i) increased latent heat release aloft in the tropics, (ii) increased dry static stability and tropopause height outside the tropics, (iii) radiative cooling of the stratosphere, (iv) Hadley cell expansion, (v) increased specific humidity following the Clausius-Clapeyron relation, (vi) cloud radiative effect changes, (vii) turbulent surface heat flux changes, and (viii) decreased surface meridional temperature gradient. I argue progress can be made by testing the thermodynamic starting points. I review recent tests of the increased latent heat release aloft in the tropics starting point, i.e., prescribing diabatic perturbations, quantifying the transient response to an abrupt CO2 increase and imposing latitudinally dependent CO2 concentration. Finally, I provide a future outlook for improving our understanding of predicted changes in the zonal mean mid-latitude circulation.

Indo-Pacific Climate Modes in Warming Climate: Consensus and Uncertainty Across Model Projections Abstract Purpose of Review Understanding the changes in climate variability in a warming climate is crucial for reliable projections of future climate change. This article reviews the recent progress in studies of how climate modes in the Indo-Pacific respond to greenhouse warming, including the consensus and uncertainty across climate models. Recent Findings Recent studies revealed a range of robust changes in the properties of climate modes, often associated with the mean state changes in the tropical Indo-Pacific. In particular, the intermodel diversity in the ocean warming pattern is a prominent source of uncertainty in mode changes. The internal variability also plays an important role in projected changes in climate modes. Summary Model biases and intermodel variability remain major challenges for reducing uncertainty in projecting climate mode changes in warming climate. Improved models and research linking simulated present-day climate and future changes are essential for reliable projections of climate mode changes. In addition, large ensembles should be used for each model to reduce the uncertainty from internal variability and isolate the forced response to global warming.

Snow and Climate: Feedbacks, Drivers, and Indices of Change Abstract Purpose of Review Highlight significant developments that have recently been made to enhance our understanding of how snow responds to climate forcing and the role that snow plays in the climate system. Recent Findings Widespread snow loss has occurred in recent decades, with the largest decreases in spring. These changes are primarily driven by temperature and precipitation, but changes in vegetation, light-absorbing impurities, and sea ice also contribute to variability. Changes in snow cover can also affect climate through the snow albedo feedback (SAF). Recently, considerable progress has been made in better understanding the processes contributing to SAF. We also highlight advances in knowledge of how snow variability is linked to large-scale atmospheric changes. Lastly, large-scale snow losses are expected to continue under climate change in all but the coldest climates. These projected changes to snow raise considerable concerns over future freshwater availability in snow-dominated watersheds. Summary The results discussed here demonstrate the widespread implications that changes to snow have on the climate system and anthropogenic activity at large.

Simulating the Midlatitude Atmospheric Circulation: What Might We Gain From High-Resolution Modeling of Air-Sea Interactions? Abstract Purpose of Review To provide a snapshot of the current research on the oceanic forcing of the atmospheric circulation in midlatitudes and a concise update on previous review papers. Recent Findings Atmospheric models used for seasonal and longer timescales predictions are starting to resolve motions so far only studied in conjunction with weather forecasts. These phenomena have horizontal scales of ~ 10–100 km which coincide with energetic scales in the ocean circulation. Evidence has been presented that, as a result of this matching of scale, oceanic forcing of the atmosphere was enhanced in models with 10–100 km grid size, especially at upper tropospheric levels. The robustness of these results and their underlying mechanisms are however unclear. Summary Despite indications that higher resolution atmospheric models respond more strongly to sea surface temperature anomalies, their responses are still generally weaker than those estimated empirically from observations. Coarse atmospheric models (grid size greater than 100 km) will miss important signals arising from future changes in ocean circulation unless new parameterizations are developed.

Amplification of Waveguide Teleconnections in the Boreal Summer Abstract Purpose of Review While the influence of climate change on mid-latitude atmospheric circulation remains uncertain, hypotheses based on linear waveguide dynamics have been proposed suggesting amplification of circumglobal quasi-stationary Rossby wave events, which may have led to persistent and high-impact extremes in recent boreal summers. It is thus useful to synthesize these hypotheses and to discuss limitations of this simplified dynamical framework for explaining observed features. Recent Findings The hypothesis that climate change can alter the basic circulation state and thereby enhance circumglobal waveguide teleconnections by increasing their resonance has been proposed but has not yet been verified with models. Furthermore, there is no convincing evidence that the variability of disturbances within the waveguide will increase in future climates projected by the CMIP5 models. On the other hand, recent research indicates that enhanced diabatic heating, particularly that associated with increasing aridity in the mid-latitude, can stimulate the jet stream waveguides, thus suggesting an alternative mechanism which, if properly modeled, could lead to more high-amplitude circumglobal planetary wave events. Summary There could be circumstances that lead to resonant amplification of waveguide Rossby waves in the boreal summer, but an alternative mechanism that involves changes in the forcing rather than the mean state deserves closer attention.

Emergent Constraints on Climate-Carbon Cycle Feedbacks Abstract Purpose of Review Feedbacks between CO2-induced climate change and the carbon cycle are now routinely represented in the Earth System Models (ESMs) that are used to make projections of future climate change. The inconclusion of climate-carbon cycle feedbacks in climate projections is an important advance, but has added a significant new source of uncertainty. This review assesses the potential for emergent constraints to reduce the uncertainties associated with climate-carbon cycle feedbacks. Recent Findings The emergent constraint technique involves using the full ensemble of models to find an across-ensemble relationship between an observable feature of the Earth System (such as a trend, interannual variation or change in seasonality) and an uncertain aspect of the future. Examples focussing on reducing uncertainties in future atmospheric CO2 concentration, carbon loss from tropical land under warming and CO2 fertilization of mid- and high-latitude photosynthesis are exemplars of these different types of emergent constraints. Summary The power of emergent constraints is that they use the enduring range in model projections to reduce uncertainty in the future of the real Earth System, but there are also risks that indiscriminate data-mining, and systematic model errors could yield misleading constraints. A hypothesis-driven theory-led approach can overcome these risks and also reveal the true promise of emergent constraints—not just as ways to reduce uncertainty in future climate change but also to catalyse advances in our understanding of the Earth System.

Regional Climate Impacts of Future Changes in the Mid–Latitude Atmospheric Circulation: a Storyline View Abstract Purpose of Review Atmospheric circulation exerts a strong control on regional climate and extremes. However, projections of future circulation change remain uncertain, thus affecting the assessment of regional climate change. The purpose of this review is to describe some key cases where regional precipitation and windiness strongly depend on the mid-latitude atmospheric circulation response to warming, and summarise this into alternative plausible storylines of regional climate change. Recent Findings Recent research has enabled to better quantify the importance of dynamical aspects of climate change in shaping regional climate. The cold season precipitation response in Mediterranean-like regions is identified as one of the most susceptible impact-relevant aspects of regional climate driven by mid-latitude circulation changes. A circulation-forced drying might already be emerging in the actual Mediterranean, Chile and southwestern Australia. Increasing evidence indicates that distinct regional changes in atmospheric circulation and European windiness might unfold depending on the interplay of different climate drivers, such as surface warming patterns, sea ice loss and stratospheric changes. Summary The multi-model mean circulation response to warming tends to show washed-out signals due to the lack of robustness in the model projections, with implications for regional changes. To better communicate the information contained within these projections, it is useful to discuss regional climate change conditionally on alternative plausible storylines of atmospheric circulation change. As progress continues in understanding the factors driving the response of circulation to global warming, developing such storylines will provide end–to–end and physically self-consistent descriptions of plausible future unfoldings of regional climate change.

The Future of Midlatitude Cyclones Abstract Purpose of Review This review brings together recent research on the structure, characteristics, dynamics, and impacts of extratropical cyclones in the future. It draws on research using idealized models and complex climate simulations, to evaluate what is known and unknown about these future changes. Recent Findings There are interacting processes that contribute to the uncertainties in future extratropical cyclone changes, e.g., changes in the horizontal and vertical structure of the atmosphere and increasing moisture content due to rising temperatures. Summary While precipitation intensity will most likely increase, along with associated increased latent heating, it is unclear to what extent and for which particular climate conditions this will feedback to increase the intensity of the cyclones. Future research could focus on bridging the gap between idealized models and complex climate models, as well as better understanding of the regional impacts of future changes in extratropical cyclones.

Climate Models as Guidance for the Design of Observing Systems: the Case of Polar Climate and Sea Ice Prediction Abstract Purpose of review The Arctic and Antarctic are among the regions most exposed to climate change, but ironically, they are also the ones for which the least observations are available. Climate models have been instrumental in completing the big picture. It is generally accepted that observations feed the development of climate models: parameterizations are designed based on empirically observed relationships, climate model predictions are initialized using observational products, and numerical simulations are evaluated given matching observational datasets. Recent findings Recent research suggests that the opposite also holds: climate models can feed the development of polar observational networks by indicating the type, location, frequency, and timing of measurements that would be most useful for answering a specific scientific question. Summary Here, we review the foundations of this emerging notion with five cases borrowed from the field of polar prediction with a focus on sea ice (sub-seasonal to centennial time scales). We suggest that climate models, besides their usual purposes, can be used to objectively prioritize future observational needs – if, of course, the limitations of the realism of these models have been recognized. This idea, which has been already extensively exploited in the context of Numerical Weather Prediction, reinforces the notion that observations and models are two sides of the same coin rather than distinct conceptual entities.

Carbon-Cycle Feedbacks Operating in the Climate System Abstract Climate change involves a direct response of the climate system to forcing which is amplified or damped by feedbacks operating in the climate system. Carbon-cycle feedbacks alter the land and ocean carbon inventories and so act to reduce or enhance the increase in atmospheric CO2 from carbon emissions. The prevailing framework for carbon-cycle feedbacks connect changes in land and ocean carbon inventories with a linear sum of dependencies on atmospheric CO2 and surface temperature. Carbon-cycle responses and feedbacks provide competing contributions: the dominant effect is that increasing atmospheric CO2 acts to enhance the land and ocean carbon stores, so providing a negative response and feedback to the original increase in atmospheric CO2, while rising surface temperature acts to reduce the land and ocean carbon stores, so providing a weaker positive feedback for atmospheric CO2. The carbon response and feedback of the land and ocean system may be expressed in terms of a combined carbon response and feedback parameter, λcarbon in units of W m− 2K− 1, and is linearly related to the physical climate feedback parameter, λclimate, revealing how carbon and climate responses and feedbacks are inter-connected. The magnitude and uncertainties in the carbon-cycle response and feedback parameter are comparable with the magnitude and uncertainties in the climate feedback parameter from clouds. Further mechanistic insight needs to be gained into how the carbon-cycle feedbacks are controlled for the land and ocean, particularly to separate often competing effects from changes in atmospheric CO2 and climate forcing.